1. Field of the Invention
The present invention relates to an image forming apparatus operable in a plurality of modes, and more specifically relates to an image forming apparatus having, for example, a plurality of developing devices installed therein so as to be capable of executing a single-color mode for forming images using a single developing device and a multi-color mode for forming images using a plurality of said developing devices.
2. Description of the Related Art
Image forming apparatus of the electrophotographic type such as copying apparatus, page printers and the like, are provided with a plurality of developing devices accommodating developing materials of different colors, which form single-color images or multi-color images (hereinafter referred to as two-color images) via operation modes corresponding to specified image colors. For example, an apparatus is disclosed in U.S. Pat. No. 4,862,216 having three functions for forming images in black, red, and both red and black.
After a plurality of developing units are operated in parallel to form two-color images, a particular developing unit may be easily contaminated by toner from another developing unit. When the developing unit containing the mixed toners of a plurality of colors is allowed to stand in said mixed state, the subsequent formation of single-color images may produce color muddiness of said images. Therefore, a so-called toner separation process must be performed to eliminate the aforesaid mixed toner.
In general, toner separation methods such as that disclosed in U.S. Pat. No. 5,063,127 set the relative electric potential difference (electrostatic contrast) between the developing units and the photosensitive member at different values during image formation. Thus, the adhesion of unnecessary toner in the developing unit to the surface of the latent image bearing member (photosensitive member) is eliminated by using different adhesion charge characteristics for toners of each color.
The toner separation process using the aforementioned method is accomplished as a so-called inter-image process within the electrophotographic process, wherein the process timing is set so as to use the region of the surface of the photosensitive member that is unaffected by the image.
In conventional copying apparatus, for example, operation setting for each portion are executed in accordance with the specified operation mode prior to the copying process, then the copy operation is started when the print key is depressed. During the copying operation, control is accomplished so as to discriminate the operation mode at the moment the toner separation process becomes executable, and when the result of said discrimination determines that the two-color image forming mode is specified, the toner separation process is executed as an inter-image process.
Conventionally, the operation mode is maintained for the toner separation process even after it is determined in the image forming process that distinguishing the operation mode for image color is unnecessary at the completion of exposure and developing.
That is, changing the operation mode is prohibited at least at the moment the toner separation process is started.
In actual practice, for, example, in the sorting mode for sorting the recording sheets (hereinafter referred to as "paper sheets") using a sorter, changing the operation mode is prohibited until a predetermined number of paper sheets have been completely discharged. In order to simplify control, the operation modes for image color are prohibited form being changed until the paper sheets are completely discharged, i.e., until the copying operation is completed.
Accordingly, when making consecutive copies in different operation modes such as, for example, copying in the single-color mode after the two-color mode, conventional methods are disadvantageous inasmuch as a long period elapses from the start of two-color mode copying until the point at which the color may be specified for the single-color copying.
Therefore, the number of copies per unit time is reduced, thereby slowing the copy speed and unavoidably lengthening the amount of time the operator must wait for the operation to be completed.